To study the effect of alterations in thyroid status on 5’-monodeiodinase activity, conversions of rT3 to 3,3’-diiodothyronine and 3’,5’-diiodothyronine (3’,5’-T2) to 3’-monoiodothyronine were examined in vitro. Rats were injected either with T4 (10 μg/100 g BW, ip, daily for 12 days) to make them thyrotoxic or thyroidectomized to render them hypothyroid, and liver and kidney homogenates were prepared. Liver homogenates from hyperthyroid animals demonstrated a 2-fold increase in 5’- monodeiodination of both rT3 and 3’,5’-T2 both reactions were also significantly increased in the kidneys of hyperthyroid rats. Hypothyroidism produced a significant decrease in 5’-deiodination of both rT3 and 3’,5’-T2 in liver and kidney homogenates. These data indicate that the in vitro 5’-deiodination of both rT3 and 3’,5’-T2 is increased in hyperthyroidism and decreased in hypothyroidism and suggest that these two iodothyronines are metabolized in a similar fashion in rat liver and kidney homogenates in states of altered thyroid function.
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